For example, as disclosed in JP-A-2012-161490, an electrocardiographic (i.e., ECG) waveform measuring apparatus, provided in a seat of a vehicle or the like, which measures an electrocardiographic waveform of a user who sits thereon includes an electrode potential detection unit having paired sensor electrodes which are disposed in a backrest portion of a seat so as to interpose a predetermined heart position therebetween at which the heart of a user who sits on the seat is located and to be parallel to a backrest surface of the seat, a guard electrode which is disposed at the outer circumferential side of each sensor electrode or the seat interior side so as not to influence a high impedance input to the sensor electrodes, and an insulating material which is disposed between these sensor electrodes and the guard electrode.
Each of the paired sensor electrode is connected to the input terminal of a sensor circuit including a known operational amplifier. Each of both the sensor circuits outputs a potential of the sensor electrode to be connected, as a detection signal, from the output terminal thereof. The detection signals which are output from both the sensor circuits are input to the input terminal of a differential amplifier, and a differential signal obtained by taking a difference between these detection signals is output from the output terminal thereof. The output differential signal is input to a band-pass filter, and is changed to an analog signal obtained by passing through a component of an electrocardiographic signal band (for example, 0.2 Hz to 35 Hz). Further, the analog signal is input to an AD converter and is converted into a digital signal. This digital signal is an electrocardiographic signal of the user who sits on the seat, and an electrocardiographic waveform obtained from the electrocardiographic signal is used in a variety of control of a vehicle in this case.
On the other hand, a circuit substrate having the sensor circuit formed thereon is housed in a hard housing case, and is disposed at a position relatively close to the corresponding sensor electrode which is located inside the backrest portion of the seat.
In the past, the electrode potential detection unit including the sensor electrode, the insulating material, and the guard electrode has been disposed slightly inside from the backrest surface of the seat. However, for the purpose of an improvement in detection sensitivity, there is a desire for disposing the electrode potential detection unit at a position closer to the backrest surface of the seat. However, the approximation of the electrode potential detection unit to the backrest surface of the seat leads to the approximation of a housing case that houses a sensor circuit substrate to the backrest surface of the seat. As a result, a problem is brought about in that sitting discomfort such as the seated user's feeling of a hard housing case on his/her back is caused.
Further, the seat of a vehicle or the like is configured such that a seat body covering material that forms a contact surface with a user who sits thereon is formed so as to cover the surface of a seat body material which is separate from the seat body covering material, but the sensor electrode is disposed relatively inside from the seat.
On the other hand, for the purpose of an improvement in detection sensitivity, there is a desire for the sensor electrode being closer to the backrest surface of the seat. However, the seat body covering material is a covered body covered with the seat body material, and position displacement between the seat body material and the covered body, position displacement between the clothes of a user or the like and the covered body, and the like occur. For this reason, there is the possibility of static charge being accumulated inside the covered body due to friction at that time. Static charge accumulated inside the covered body is locally discharged to an exposed surface (electrode surface) of the sensor electrode at the moment when relative position movement or pressure such as traveling vibration or pressure of the seat body covering material caused by a user who sits on the seat is generated between the seat body covering material and the sensor electrode, and is superimposed on an electrocardiographic signal as electric noise, which leads to the possibility of a deterioration in detection accuracy being caused.